Phototrophic organisms need to ensure high photosynthetic performance whilst suppressing reactive oxygen species (ROS)-induced stress occurring under excess light conditions. The xanthophyll cycle (XC), related to the high-energy quenching component (qE) of the nonphotochemical quenching (NPQ) of excitation energy, is considered to be an obligatory component of photoprotective mechanisms. The pigment composition of at least one representative of each major clade of Ulvophyceae (Chlorophyta) was investigated. We searched for a light-dependent conversion of pigments and investigated the NPQ capacity with regard to the contribution of XC and the qE component when grown under different light conditions. A XC was found to be absent in a monophyletic group of Ulvophyceae, the Bryopsidales, when cultivated under low light, but was triggered in one of the 10 investigated bryopsidalean species, Caulerpa cf. taxifolia, when cultivated under high light. Although Bryopsidales accumulate zeaxanthin (Zea) under high-light (HL) conditions, NPQ formation is independent of a XC and not related to qE. qE- and XC-independent NPQ in the Bryopsidales contradicts the common perception regarding its ubiquitous occurrence in Chloroplastida. Zea accumulation in HL-acclimated Bryopsidales most probably represents a remnant of a functional XC. The existence of a monophyletic algal taxon that lacks qE highlights the need for broad biodiversity studies on photoprotective mechanisms.

German Academic Exchange Service (DAAD); Foundation of Science and Technology (FCT) [IF/00899/2014/CP1222/CT0010, BPD/102572/2014]; German Research Foundation (DFG) [GO1825/4-1, JA665/11-1, VR 132/1-1]; Centre for Environmental and Marine Studies [UID/AMB/50017]; FCT/Ministry of Science and Education; European Fund For Regional Development; European Research Council [ERC 666053]

funding text

Financial support for this work was provided by the German Academic Exchange Service (DAAD) to G.C. (P.R.I.M.E.), the Foundation of Science and Technology (FCT) to S.C. (IF/00899/2014/CP1222/CT0010) and A.C.E. (BPD/102572/2014), and the German Research Foundation (DFG) to S.B.G. (GO1825/4-1), P.J. (JA665/11-1) and J.d.V. (VR 132/1-1). For financial support, thanks are due to Centre for Environmental and Marine Studies (UID/AMB/50017), FCT/Ministry of Science and Education through national funds, and the co-funding by European Fund For Regional Development, within the PT2020 Partnership Agreement and Compete 2020. We thank Margarethe Stracke, Angela Doevenspeck-Schuler, Silja Frankenbach and William Schmitt for help with measurements and algal culture, and William F. Martin for infrastructure and financial support (European Research Council, ERC 666053) of our work.